Plasma chambers commonly are used to perform processes for fabricating electronic devices such as semiconductors, displays and solar cells. Such plasma fabrication processes include chemical vapor deposition of semiconductor, conductor or dielectric layers on the surface of a workpiece, or etching of selected portions of such layers on the workpiece surface.
FIG. 1 shows a conventional plasma chamber. The workpiece 10 is supported on a susceptor 12 within the chamber. In performing a plasma process on the workpiece, one or more process gases are dispensed into the chamber through a gas inlet manifold 20-26. The gas inlet manifold includes a manifold back wall 20, a showerhead 22 (also called a gas distribution plate or diffusor), and a suspension 24, all of which collectively enclose a volume which is the interior 26 of the gas inlet manifold.
A gas inlet conduit 28 extends through the center of the manifold back wall 20. A gas source, not shown, supplies process gases to the upper end of the gas inlet conduit. The process gases flow from the gas inlet conduit into the interior 26 of the gas inlet manifold, and then are dispensed into the interior 11 of the plasma chamber through numerous gas passageways in the showerhead 22.
The gas inlet manifold 20-26 also functions as an electrode to couple RF power from an RF power supply to a plasma in the interior 11 of the plasma chamber between the showerhead and the susceptor. The manifold back wall 20, showerhead 22, suspension 24 and gas inlet conduit 28 are electrically conductive. A first RF cable 36 couples RF power from the output of an RF power supply 32 to an impedance matching network 34. A second RF cable 30 couples RF power from the impedance matching network 34 to the gas inlet conduit 28, which functions as an RF input 40 of the plasma chamber.
The gas inlet conduit 28 is electrically connected to the center of the manifold back wall. RF power flows radially outward through the manifold back wall from the gas inlet conduit at the center of the manifold back wall to the four suspension walls 24 at each of the four sides of the manifold back wall, and then through the four suspension walls to the four sides of the showerhead 22. The RF power is coupled from the showerhead to the plasma in the plasma chamber interior 11 between the showerhead and the susceptor 12.
A shortcoming of this conventional RF power connection design is that the complex impedance of the electrical load presented to the RF input 40 (i.e., where the second RF cable 30 electrically connects to the gas inlet conduit 28) typically has an inductive component significantly greater than its resistive component, which produces high peak voltages in the gas inlet manifold 20-26, the impedance matching network 34, and the RF circuitry connected between them. Such high peak voltage is undesirable because it can cause atmospheric arcing (i.e., electrical discharge) in the portions of the RF circuitry that are exposed to atmosphere, and it can cause failure of the capacitors within the RF circuitry.